1. Field of the Invention
The invention relates to cancellation of vibration in electrically powered devices, and in particular to a method and apparatus for reducing vibration and acoustic noise in an AC powered device by feedback of a corrective cancelling vibration related to the AC power line frequency.
2. Prior Art
Electrically powered devices vibrate due to imbalances such as eccentricities in rotating elements and translation of movable elements, and also due to periodic displacement of elements that normally are regarded as being stationary, such as the housing of a pump or motor or the foundation on which the device is mounted. One form of displacement in a rotating machine occurs at the rotation frequency. Another form of displacement occurs as a result of electromagnetic forces, which are at the power line frequency.
The electrical line frequency of a motor or the like is typically the frequency of the power grid, i.e., 60 Hz in the US or 50 Hz in Europe. An AC motor also may be coupled to a speed controller that produces a driving signal of variable frequency. For purposes of this disclosure, the electrical driving frequency of the motor is in any case termed the line frequency, it being understood that the electrical driving frequency may be different than the power line frequency that feeds the speed controller or the like.
The line frequency of a generator is the frequency of the signal generated by the machine, typically an integral multiple of the rotational frequency. Electrical and electronic equipment also vibrate at multiples or harmonics of their line frequency. This vibration produces acoustical noise that can be objectionable, especially in large or high powered devices.
In many cases, the vibration of equipment at multiples of the line frequency is more objectionable than vibration at the line frequency. For example, transformer "hum" occurs at twice the line frequency (i.e., 120 Hz) and can be annoying. The vibration and resulting acoustic noise are objectionable from the standpoint of occupational safety and health, especially for large transformers. Also, in rotating machines, vibrations at the rotating frequency (and at harmonics thereof) may beat against the line frequency and its harmonics, producing vibrations and noise at sideband frequencies related to the sums and differences of the respective frequencies. For example, a generator or motor may vibrate at and at multiples of the rotating frequency plus and minus one, two or more times the line frequency. The vibration and noise levels at these sideband frequencies can be objectionable for various reasons.
Depending on the element that is displaced (vibrated) in producing noise, one can attempt to limit vibration at various specific places in the noise producing device. For a transformer producing objectionable hum, all large plate structures radiating noise may require vibration elimination structures. A pump such as the coolant pump of a nuclear reactor may require structures attached to the motor pump housing. A generator or motor may require elimination of vibration coupled from the rotor to the foundation structures on which the device is mounted.
One means for dealing with vibration at rotational frequencies is to balance or support the rotating element of a generator or motor via electromagnetic bearings. The position of the rotor is sensed and applied to a feedback circuit. The feedback signal is applied via power amplifiers to modulate the balancing signal applied to the electromagnetic bearings to cancel vibration at the rotational frequency. Examples of this technique are disclosed, for example, in U.S. Pat. Nos. 4,999,534--Andrianos and 4,626,754--Habermann et. al. Generally, the correction signal applied to the bearings is in synch with the vibration frequency and out of phase. The correction signal in Habermann '754 is developed using one or several feedback loops. In Andrianos the correction signal is developed using adaptive controllers.
It is also known to cancel rotational vibrations by attaching to the vibrating element an electromagnetic shaker having a movable mass (i.e., a mass apart from the rotor or the like), and electromagnetically vibrating the mass. The mass is displaced in synch and out of phase with the vibration of the vibrating element. The forces producing vibration of the element are counteracted by the force produced by the vibrating mass. These forces are summed because the shaker and the vibrating element are attached. Examples of this technique are disclosed in U.S. Pat. Nos. 4,963,804--Geiger; 4,947,067--Habermann et. al.; 4,922,159--Phillips et. al.; 4,083,433--Geohegan, Jr. et. al.; and 3,088,062--Hudimac. Geiger and Habermann '067 employ shakers with a floating mass, thereby reducing friction, and displace the mass along three mutually perpendicular axes. Phillips and Hudimac teach particular feedback control couplings and equations for generating a correction signal. Each uses a circuit that is tuned to a particular vibration frequency, and therefore is arranged for operation at a particular rotation frequency of the machine.
It is possible to sum feedback signals from a number of parallel tuned circuits to handle harmonics. However, this is impractical if the number of frequencies to be handled becomes large. Such tuned circuits are not practical for a variable speed device, and in fact may introduce instabilities.
According to the present invention a correction signal is provided to handle vibration at the line frequency and its harmonics. The invention can be used in conjunction with a feedback apparatus operable at rotational frequencies as in U.S. Pat. No. 4,963,804--Geiger, to further cancel vibration by providing cancellation related to the line frequency applied to the pump motor. In that case, the components of the cancellation signals at rotational and line frequencies are summed or beat together in the same manner that the offending line and rotational vibratory frequencies and their harmonics are summed. The apparatus thus counteracts vibration at the line and rotational frequencies, at their harmonics, and at the sideband frequencies defined by their sums and differences.
To cancel vibration that is directed parallel to the rotation axis, lateral of the axis and rotationally around the axis, four shaker mechanisms are provided, each arranged for producing an oscillatory force in three mutually perpendicular axes. The four shaker mechanisms preferably are arranged at 90.degree. intervals around the rotation axis of a rotating machine, being simply attached on the outside of the machine housing.
Accordingly, the invention is particularly applicable to controlling vibration of the liquid coolant pump of a nuclear reactor. No internal changes are needed in the pump and there is no requirement for additional penetration of the pressure barrier enclosing the coolant circuit. The shaker mechanisms are not subjected to extremes of heat and pressure or to exposure to the coolant fluid. The invention can counteract vibration over variable line frequencies and rotational speeds, and readily can be retrofit to a pump or the like.